Wireless local area network (WLAN) systems enable the communication of data via a wireless communication medium by, for example, transmitting radio frequency (RF) signals that carry data between a transmitting station and a receiving station. A range of frequencies, referred to as the WLAN frequency spectrum, may be utilized for communication between stations in a WLAN system. The frequency spectrum may be divided into RF channels wherein each RF channel represented an assigned frequency within the WLAN frequency spectrum. Each RF channel may, in turn, comprise a range of frequencies referred to as an RF channel bandwidth. Each RF channel within the WLAN frequency spectrum may comprise a range of frequencies, which is non-overlapping and distinct from other RF channels.
In a typical WLAN setting there are various objects present in addition to the transmitting station and the receiving station. The transmitting station transmits data to the receiving station via data symbols, which are transmitted via a transmitted signal. The transmitted signal may comprise a one or more frequency carrier signals (wherein each frequency carrier signal is associated with a distinct frequency within a given RF channel bandwidth), which are utilized to generate a corresponding one or more carrier-modulated signals to enable the transport of the data symbols via the wireless communication medium. The time interval, beginning at the time instant at which the transmitting station begins transmission one or more current data symbols via the one or more frequency carrier signals, and ending at the time instant at which the transmitting station begins transmission of a subsequent one or more data symbols may be referred to as a symbol period.
Signals transmitted by the transmitting stations typically experience a natural expansion of the radio wave front as the signals propagate in the wireless communication medium. Portions of the expanding signal often interact with the various objects present in the WLAN setting and are, in many cases, reflected off the various objects. The reflected signal portions may continue propagating in the wireless communication medium. One or more portions of a transmitted signal may experience multiple reflections while propagating through the wireless communication medium. Each of the one or more portions of the transmitted signal is referred to as a multipath signal. The path traveled by a multipath signal may be referred to as a signal path.
A plurality of multipath signals may be received at the receiving station. The multipath signals may comprise a line of sight (LOS) signal, which is transmitted from the transmitting station, via the wireless communication medium, to the receiving station without encountering reflections. In addition, the multipath signals may comprise one or more signals, which encounter one or more reflections while propagating from the transmitting station to the receiving station via the wireless communication medium. The various multipath signals, which are received at the receiving station, may arrive at different time instants. The time interval, beginning at the time instant at which the first of the multipath signals arrives at the receiving station and ending at the time instant at which the last of the multipath signals arrives at the receiving station is referred to as a delay spread.
In addition to delay spread that results from multiple signal paths, there may also be a delay spread within a given signal path. For example, in WLAN systems, which utilize orthogonal frequency division multiplexing (OFDM), an OFDM symbol may be generated by concurrently transmitting individual data symbols via a plurality of concurrently transmitted frequency carrier signals. Delay spread may occur within a given signal path when some of the frequency carrier signals within an RF channel bandwidth propagate through the wireless communication medium at different speed(s) relative to other frequency carrier signals. The delay spread may be utilized to determine the coherence bandwidth for the RF channel.
Signals, which are transmitted from a transmitting station to a receiving station, are typically subjected to distortion as they are propagated through the wireless communication medium. Consequently, the receiving station may receive a distorted version of the signals transmitted by the transmitting station. The distortion of transmitted signals is referred to as fading. Two types of fading are flat fading and frequency selective fading. Flat fading may occur when the delay spread is less than the symbol period, or correspondingly, when the signal bandwidth for the RF channel is less than the coherence bandwidth. In a flat fading RF channel, amplitude fading may be a contributor to the signal fading in the RF channel. Amplitude fading refers to the tendency of signals to attenuate as they are propagated through a wireless communication medium. Frequency selective fading may occur when the delay spread is greater than the symbol period, or correspondingly, when the signal bandwidth for the RF channel is greater than the coherence bandwidth. In a frequency selective fading RF channel, intersymbol interference may be a contributor to signal fading in the RF channel. Intersymbol interference refers to an occurrence in which a receiving station begins to receive signals for a current one or more transmitted data symbols while still receiving signals from a previous one or more transmitted data symbols.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.